Human locomotion, such as walking and running, is commonly described in terms of gait. Gait is a cyclical or reoccurring pattern of leg and foot movement, rotations, and torques that creates locomotion. Due to the repetitive nature of gait, gait is typically analyzed in terms of percentages of a gait cycle. A gait cycle is defined for a single leg beginning with the initial contact of the foot with a surface such as the ground. The initial contact of the foot on the ground is referred to as a heel strike. The conclusion of a gait cycle occurs when the same foot makes a second heel strike. A gait cycle can be divided into two phases: stance phase and swing phase. Stance phase describes the part of the gait cycle where the foot is in contact with the ground. Stance phase begins with heel strike and ends when the toe of the same foot leaves the ground. Swing phase describes the part of the gait cycle where the foot is in the air and not in contact with the ground. Swing phase begins when the foot leaves contact with the ground and ends with the heel strike of the same foot. For walking gait speed, stance phase typically describes the first 60% of the gait cycle, while swing phase describes the remaining 40% of the gait cycle.
Prosthetic and orthotic devices help restore mobility to people who lack able-bodied motion or gait. Prosthetic devices are intended to replace the function or appearance of a missing limb and can return mobility to the wearer or user. Orthotic devices are intended to support or supplement an existing limb, by assisting with movement, reducing weight-bearing loads on the body, reducing pain, and controlling or restricting movement. Prosthetic and orthotic devices are available to replace or support various portions of the body. Lower limb prosthetic devices include, for example, the prosthetic foot, the foot-ankle prosthesis, the prosthetic knee joint, and the prosthetic hip joint. Lower limb orthotic devices include, for example, the foot orthoses, the ankle-foot orthoses, the knee-ankle-foot orthoses, and the knee orthoses. People who require a lower limb prosthesis or orthosis often expend more metabolic power to walk or move at the same speed as able-bodied individuals. One goal of lower limb prosthetic and orthotic devices is to help the user achieve a normal gait while reducing energy expended by the user.
The gait dynamics of a human joint can be described in terms of the position, velocity, moment, and power. During a typical walking gait cycle, the moment required from a human ankle reaches a maximum value of approximately 1.25 Newton meters per kilogram (N-m/kg) of body weight, while the typical velocity reaches a maximum of approximately 215 degrees per second, and the maximum power reaches approximately 3.5 Watts per kilogram (W/kg) of body weight. One goal of prosthetic and orthotic devices is to match the characteristics of able-bodied gait.
Prosthetic and orthotic devices can be divided into three groups, passive devices, active devices, and bionic devices. Passive lower limb prosthetics generally rely on compliant members, such as springs, to store and release energy. A spring is able to return only as much energy as is put into the spring, minus efficiency losses. Thus, the energy that is released by a spring in a passive device is limited to the energy that is put in by the user. Additionally, existing spring-based prosthetic ankles return the energy inefficiently to the user and are optimized for a single gait speed. As result, current prosthetic ankles can lack sufficient power return to produce normal gait. The user of a prosthetic must expend additional energy through recruiting other muscles and joints in a compensation strategy to maintain a functional gait. Therefore, passive prosthetic and orthotic designs are limited in capacity to reduce a user's metabolic energy expenditure while achieving a normal walking gait and performing other activities. Existing research has shown a 10-30% increase in metabolic cost for walking over able-bodied norms, depending on amputation level and gait speed.
Active devices differ from passive devices in that active devices employ a microprocessor and actuator to supply power to the device and to control the device. One type of active lower limb prosthetic device uses a microprocessor to control damping characteristics. Damping is typically performed with hydraulic valves and has the effect of converting energy into waste heat. Other active devices use a motor and drive transmission to control the orientation of the foot body relative to the shank body.
Fully active or bionic devices differ from active devices in that bionic devices employ a motor to supply power to the device and to control the device and add energy to the user. Current bionic devices face many design challenges. Some bionic device designs attempt to fully power knee or ankle gait. Bionic devices require larger motors, heavier and more robust drive-trains, larger and heavier batteries, struggle to provide enough power output for moderate gait activities.
Control systems for bionic devices are limited in capability to control the devices, because the systems require a signature gait move to occur before triggering a controller to switch gait activities, such as ascending or descending slopes or stairs. Further, bionic prostheses are limited to low or moderate power gait activities, because the power output necessary for high power gait activities such as running or jumping are not sustainable in a small portable system. One goal of bionic device designs is to increase efficiency of the active components and to build a lighter weight and more intuitively controlled system.
Another goal of prosthetic device designs is to perform more similarly to a human muscle during a variety of activities. Prosthetic devices are typically designed for a specific activity, such as walking. The majority of active compliant devices utilize a traditional rigid structure. The traditional rigid structure typically includes links powered by actuators such as electric motors or hydraulics. An activity-specific design strategy and traditional rigid structures may be suited for one specific activity, but the designs are limited in application and are not efficient beyond the intended activity. For example, devices designed for walking perform poorly for running, navigating uneven terrain, walking up and down inclines or stairs, or simply balancing while standing. Carrying heavy loads or transitioning from walking to running remains a challenge for users.